This invention relates to processes for making anhydrous magnesium chloride.
One important use of the product of the present process is in the electrolytic production of magnesium metal from magnesium chloride, for which anhydrous magnesium chloride of high purity is required. Magnesium chloride occurs in natural and artificial brines, and in ores such as carnallite and bischofite, in a polyhydrated form, e.g. as hexahydrate. Suitable thermal techniques are known for removing some of this water of hydration, to achieve the dihydrate; but procedures heretofore employed to effect the requisite virtually complete removal of water have been attended with various difficulties.
It is known that ammoniated magnesium chloride compounds are usable as material for producing anhydrous magnesium chloride. Ammoniation is often carried out under anhydrous conditions as done for example in U.S. Pat. No. 3,966,888. More particularly, U.S. Pat. No. 3,966,888 describes a process in which hydrated forms of MgCl.sub.2 are dissolved in ethylene glycol. The water is then distilled off from this solution and after subsequent cooling to a temperature between 15.degree. and 50.degree. C., the anhydrous solution is reacted with gaseous ammonia to form magnesium chloride hexammoniate, MgCl.sub.2.6NH.sub.3, which is washed with anhydrous methanol and then decomposed to anhydrous magnesium chloride. Other processes where the ammoniation is carried out under anhydrous conditions have been proposed. For example, in U.S. Pat. No. 2,381,994 a hydrous form of magnesium chloride is dissolved in monohydroxy aliphatic alcohols having boiling points between 100.degree. and 180.degree. C., and is precipitated as MgCl.sub.2.6NH.sub.3 after the removal of water by distillation. In the process of U.S. Pat. No. 3,352,634 partially dehydrated MgCl.sub.2 is dissolved in a an alcohol containing from 4 to 10 carbon atoms; the solution is then subjected to azeotropic drying to give a bone dry solution of MgCl.sub.2 in the alcohol, and thereafter treated with ammonia to precipitate MgCl.sub.2.6NH.sub.3. The problem with such schemes is that a second alcohol, of low boiling point temperature such as methanol, is required for the washing of the ammoniate precipitate since, otherwise, the high boiling point alcohol decomposes, leaving a carbon residue. Other common problems are the very high consumption of ammonia, with up to 30 moles NH.sub.3 required to produce 1 mole of anhydrous MgCl.sub.2 and the low precipitation yields of MgCl.sub.2.6NH.sub.3 due to the high solubility of the ammoniate compound in the high boiling point alcohol.
Ammoniation has been also carried out in aqueous reacting media. For example, U.S. Pat. No. 3,092,450 discloses a process for producing MgCl.sub.2.6NH.sub.3 by adding an aqueous solution of magnesium chloride and ammonium chloride to an aqueous solution containing ammonia, at low temperatures between -20.degree. and 10.degree. C., so as to allow magnesium chloride hexammoniate to precipitate. Another variant of the process, described in U.S. Pat. No. 4,228,144, proposes to carry the ammoniation reaction at temperatures varying between -50.degree. and 0.degree. C. The problem with such schemes is that the reaction yields are relatively low, i.e 60-70%. Therefore, large amounts of unreacted magnesium chloride in reaction mixture have to be recovered and reused. Also, very high amounts of ammonia are required, i.e. 20-30 moles of NH.sub.3 per mole of magnesium chloride produced. Finally, the reaction has to be carried out at low temperatures, i.e. typically -50.degree. C. to 0.degree. C. in the case of U.S. Pat. No. 4,228,144. Since the ammoniation reaction is an exothermic reaction, the operation at low temperatures requires a large amount of refrigeration which makes the process expensive. To partially overcome this difficulty, the process described in U.S. Pat. No. 3,092,450 proposes to carry out part of the ammoniation step at higher temperatures but this requires the use of pressures substantially above atmospheric.
A variety of processes for purifying magnesium chloride values have also been proposed. U.S. Pat. No. 3,357,800 describes a process in which impure MgCl.sub.2 containing alkali metal halides is dissolved in alcohols, preferably methanol, and after filtration to remove insoluble impurities, magnesium chloride hydrate is recovered by evaporation of the solvent. In the process of U.S. Pat. No. 4,056,599, carnallite, MgCl.sub.2.KCl.6H.sub.2 O, is dissolved in methanol to make a solution from which the insoluble impurities and KCl are separated; thereafter the solution is heated to distil off the solvent, leaving behind an aqueous solution of pure MgCl.sub.2.